Method for producing spherical silica aerogel powder

Abstract

This invention provides a method for producing silica aerogel powder that reduces the silylation treatment time using siloxanes, which previously accounted for the majority of the manufacturing time. [Solution] A method for producing silica aerogel powder, characterized in that when performing a silylation treatment by adding siloxanes and an acid catalyst to a mixed solvent of water and a hydrophilic organic solvent in which the gelled material is dispersed, the siloxanes and concentrated sulfuric acid are mixed together in advance to form a mixed solution, and then the mixed solution of siloxanes and concentrated sulfuric acid is added to the mixed solvent of water and a hydrophilic organic solvent in which the gelled material is dispersed to perform the silylation treatment.

Description

Technical Field

[0001] The present invention relates to a method for producing spherical silica aerogel powder.

Background Art

[0002] An aerogel is a material having a high porosity and excellent oil absorbency. The aerogel referred to here means a solid material having a porous structure and accompanied by a gas as a dispersion medium, particularly a solid material having a porosity of 60% or more. The porosity is a value obtained by expressing the amount of gas contained in the apparent volume as a volume percentage. Due to the high porosity, the aerogel has excellent oil absorbency.

[0003] The uses of silica aerogel are various, such as cosmetic materials, heat insulating fillers, matting agents for paints, abrasive grains for semiconductor polishing agents, etc. For example, when used as a cosmetic material, taking foundation as an example, it is used as an additive for improving the appearance persistence when applied to the skin. More specifically, the porous structure of silica aerogel can well absorb sebum, so that it can prevent the skin from getting wet with sebum and increasing the specular reflectance of light to cause greasiness. Moreover, when silica aerogel is manufactured by hydrophobization, its affinity with the organic components of cosmetic materials such as foundation becomes good and it is uniformly dispersed, so that the appearance persistence effect of preventing greasiness is further enhanced.

[0004] When these silica aerogels are blended into cosmetics, in order to obtain a smooth touch feeling, the particle size is 1 to several tens of μm, and in order to improve the rolling property on the skin, it is desirable that the shape is spherical.

[0005] Furthermore, silica aerogel is also useful as an insulating filler. Heat conduction within an object is contributed to by solid conduction (propagation of thermal vibrations), convection, and radiation, and in materials with a large porosity, convection generally contributes the most. In contrast, silica aerogel has a very small pore size of about 10 to 100 nm, so the movement of gas in the voids is greatly restricted, and heat conduction by convection is significantly inhibited. For this reason, silica aerogel has excellent insulating properties. Generally, the insulating performance depends on the amount of filler added to the substrate, so in order to increase the filling density of silica aerogel and improve its insulating performance, it is desirable for silica aerogel to be spherical.

[0006] As a method for producing such spherical silica aerogels with an appropriate particle size, the following method has been proposed.

[0007] Patent Document 1 discloses a method for producing spherical silica aerogel, comprising the steps of: preparing an aqueous silica sol; dispersing the aqueous silica sol in a hydrophobic solvent to form a W / O emulsion; gelling the silica sol to convert the W / O emulsion into a dispersion of the gelled body; replacing the water in the gelled body with a solvent having a surface tension of 30 mN / m or less at 20°C; hydrophobizing (silylation) the gelled body with a hydrophobic agent (silylation agent); and removing the substituted solvent, in the order described above.

[0008] Patent Document 2 discloses a method for producing spherical silica aerogel, which sequentially includes the steps of: separating the dispersion of the gelled substance obtained in the step of converting the W / O type emulsion into a dispersion of the gelled substance into two layers, an O phase and a W phase; adding a basic substance to the W phase to mature the gelled substance dispersed in the W phase; silylation treatment of the gelled substance dispersed in the W phase; extracting the gelled substance with a hydrophobic organic solvent; and recovering the gelled substance to obtain a powder consisting of hydrophobic spherical silica aerogel. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] International Publication No. 2012 / 057086 [Patent Document 2] Japanese Patent Publication No. 2018-177620 [Overview of the project] [Problems that the invention aims to solve]

[0010] In the hydrophobic treatment of gelled bodies, siloxanes such as hexamethyldisiloxane and octamethylcyclotetrasiloxane are widely used. It is believed that the reactivity is improved by adding catalysts such as acids and bases, or by contacting the gelled body with siloxanes in a mixed solvent of water and hydrophilic organic solvent. However, Patent Document 2 requires 12 hours for the silylation treatment, and from the standpoint of economic rationality, further reduction of the silylation treatment time is desired. [Means for solving the problem]

[0011] As a result of diligent research to solve the above problems, the inventors of the present invention have found that in a method for producing silica aerogel powder, when silica powder and siloxanes are brought into contact in a mixed solvent of water and a hydrophilic organic solvent for silylation treatment, the reaction in which the siloxane bonds of the siloxanes are cleaved affects the silylation treatment time.

[0012] Based on the above findings, further research revealed that when performing a silylation treatment by adding siloxanes and an acid catalyst to a mixed solvent of water and a hydrophilic organic solvent in which a gelled material is dispersed, the silylation treatment reaction is accelerated by first mixing the siloxanes with concentrated sulfuric acid to form a mixed solution before adding it to the mixed solvent of water and a hydrophilic organic solvent in which the gelled material is dispersed.

[0013] Furthermore, we found that siloxanes can be extracted into concentrated sulfuric acid from a hydrophobic organic solvent containing siloxanes generated in the process after silylation, and that hydrating the concentrated sulfuric acid containing the extracted siloxanes releases the siloxanes, making them available for isolation.

[0014] Based on these findings, we have completed the present invention.

[0015] In other words, the present invention is (1) Steps to prepare aqueous silica sol (2) A step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W / O type emulsion. (3) A step of converting the W / O type emulsion into a dispersion of the gelled product. (4) A step of separating the dispersion into two layers, an O phase and a W phase. (5) Step to remove the O phase (6) A step of silylation treatment of the gelled material dispersed in the W phase with a mixture of concentrated sulfuric acid and siloxanes. (7) Step of extracting the gelled product that has been silylated with a hydrophobic organic solvent. (8) A step to recover silica gel and obtain hydrophobic spherical silica aerogel. This is a method for producing silica aerogel powder containing [the specified ingredient].

[0016] The present invention further includes a step of recovering siloxanes from a hydrophobic organic solvent solution containing siloxanes generated in the step of recovering silica gel (9)(8) to obtain a hydrophobic spherical silica aerogel. Preferably, this step of recovering siloxanes from a hydrophobic organic solvent solution containing siloxanes generated in the step of recovering silica gel (9)(8) to obtain a hydrophobic spherical silica aerogel is a step of recovering siloxanes by extracting them from the hydrophobic organic solvent solution containing siloxanes with concentrated sulfuric acid. By extracting siloxanes with concentrated sulfuric acid, the siloxanes may be recovered as a mixture of concentrated sulfuric acid and siloxanes, or further, the siloxanes may be recovered by adding this mixture of concentrated sulfuric acid and siloxanes to water. Siloxanes recovered as a mixture with concentrated sulfuric acid or as siloxanes can be used as at least a portion of the mixture of concentrated sulfuric acid and siloxanes or siloxanes in the step of silylation treatment of a gelled body dispersed in the W phase with a mixture of concentrated sulfuric acid and siloxanes. [Effects of the Invention]

[0017] The method for producing the silica aerogel powder of the present invention accelerates the silylation rate using siloxanes. Therefore, the production time of the hydrophobic silica powder can be shortened. Furthermore, since the unreacted portion of the used siloxanes can be recovered, it becomes easy to reuse.

Embodiments for Carrying Out the Invention

[0018] The method for producing the silica aerogel powder of the present invention includes the following steps. That is, (1) Step of preparing an aqueous silica sol (2) Step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W / O type emulsion (3) Step of converting the W / O type emulsion into a dispersion of a gelified product (4) Step of separating the dispersion into two layers of an O phase and a W phase (5) Step of removing the O phase (6) Step of subjecting the gelified product dispersed in the W phase to silylation treatment with a mixed solution of concentrated sulfuric acid and siloxanes (7) Step of extracting the gelified product silylated with a hydrophobic organic solvent (8) Step of recovering silica gel and obtaining hydrophobic spherical silica aerogel That's it.

[0019] <(Step of preparing an aqueous silica sol)> As a raw material for the silica sol, a method using an alkali metal silicate or the like can be preferably adopted because it is inexpensive. Examples of the alkali metal silicate include potassium silicate, sodium silicate, etc., and the composition formula is represented by the following formula (1).

[0020] m(M2O)·n(SiO2) (1) [In formula (1), m and n each independently represent a positive integer, and M represents an alkali metal atom.] Among the raw materials for preparing the above silica sol, sodium silicate, which is easily available, is particularly suitable.

[0021] The following explanation will use alkali metal silicate salts as a raw material as an example.

[0022] When using an alkali metal silicate as a raw material for preparing the aqueous silica sol of the present invention, it is preferable to prepare the silica sol by neutralizing it with a mineral acid such as hydrochloric acid or sulfuric acid. Specifically, this can be done by adding an aqueous solution of the alkali metal silicate to an aqueous solution of the acid while stirring the solution, or by causing a collision mix of the aqueous solution of the acid and the aqueous solution of the alkali metal silicate in a pipe (see, for example, Japanese Patent Publication No. 4-54619).

[0023] In this invention, the pH of the prepared silica sol is set to be in the acidic range. Specifically, the amount of acid used when preparing the aqueous silica sol is preferably such that the molar ratio of hydrogen ions to alkali metal content in the alkali metal silicate salt is 1.05 to 1.2. When the amount of acid is within this range, the pH of the prepared silica sol will be approximately 1 to 5. More preferably, the amount of acid is adjusted so that the pH of the prepared silica sol is 2.5 to 3.5.

[0024] The silica sol prepared by the above method is preferably concentrated at 50 g / L or more in terms of silica content (SiO2 equivalent concentration), as this allows gelation to be completed in a relatively short time, sufficiently forms a skeletal structure of silica particles to suppress shrinkage during drying, and makes it easier to obtain a large pore volume. On the other hand, it is preferable to have a silica concentration of 160 g / L or less, and more preferably 100 g / L or less, as this allows for a relatively low density of silica particles, resulting in a good pore volume and easier oil absorption. Even more preferably, it is 90 to 100 g / L.

[0025] By setting the concentration of aqueous silica sol to above the lower limit, it becomes easier to set the pore volume of the aerogel by the BJH method to 8 mL / g or less, and also to set the peak of the pore radius of the aerogel by the BJH method to 50 nm or less. Furthermore, by setting the concentration of aqueous silica gel to below the upper limit, it becomes easier to set the pore volume of the aerogel by the BJH method described in the above patent document to 2 mL / g or more, and also to set the peak of the pore radius of the aerogel by the BJH method to 10 nm or more.

[0026] In this invention, the pore volume obtained by the BJH method is obtained by drying the sample to be measured at a temperature of 150°C for 2 hours or more under a vacuum of 1 kPa or less, then obtaining the adsorption isotherm only on the nitrogen adsorption side at liquid nitrogen temperature, and analyzing it using the BJH method (Barrett, EP; Joyner, LG; Halenda, PP, J. Am. Chem. Soc. 73, 373 (1951)). The pores measured by this method are pores with a radius of 1 to 100 nm, and the integrated value of the volume of pores in this range is the pore volume in this invention.

[0027] (2) A step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W / O type emulsion. In the manufacturing method of the present invention, the aqueous silica sol obtained by the method described above is dispersed in a hydrophobic solvent to form a W / O emulsion. By forming such a W / O emulsion, the silica sol becomes spherical due to surface tension, etc., and by gelling the silica sol dispersed in the hydrophobic solvent in this spherical shape, a spherical gel can be obtained. In this way, by going through the emulsion formation step of forming a W / O emulsion, it becomes possible to produce aerogels that usually have a high circularity of 0.8 or more.

[0028] The hydrophobic solvent can be any solvent that is hydrophobic enough to form a W / O emulsion with aqueous silica sol. Suitable solvents include, for example, organic solvents such as hydrocarbons and halogenated hydrocarbons. More specifically, examples include hexane, heptane, octane, nonane, decane, dichloromethane, chloroform, carbon tetrachloride, and dichloropropane. Among these, heptane, which has a suitable viscosity, is particularly preferable. Multiple solvents may be mixed and used as needed. Furthermore, water-soluble solvents such as lower alcohols can be used in combination (as a mixed solvent), as long as they can form a W / O emulsion with aqueous silica sol.

[0029] The amount of hydrophobic solvent used is not particularly limited, as long as it is enough to form a w / o emulsion. However, generally, an amount of hydrophobic solvent of about 1 to 10 parts by volume is used per 1 part by volume of aqueous silica sol.

[0030] When forming the above W / O emulsion, it is preferable to add a surfactant. Any of anionic surfactants, cationic surfactants, or nonionic surfactants can be used. Among these, nonionic surfactants are preferred because they readily form a W / O emulsion. In the present invention, since the silica sol is aqueous, a surfactant with an HLB value of 3 or more and 6 or less, which indicates the degree of water solubility and hydrophobicity of the surfactant, can be suitably used. In the present invention, "HLB value" refers to the HLB value by the Griffin method.

[0031] As described above, in the present invention, the shape of the aerogel particles is largely determined by the shape of the droplets of the W / O emulsion. The shape of the droplets depends on the surfactant used. As stated above, the shape of the aerogel particles is preferably spherical, and from this viewpoint, specific surfactants that can be suitably used include sorbitan monooleate, sorbitan monostearate, sorbitan monosesquioleate, and the like.

[0032] The amount of surfactant used is no different from the typical amount used to form a W / O emulsion. Specifically, a range of 0.05 g to 10 g per 100 ml of aqueous silica sol is preferably used. If the amount of surfactant used is high, the droplets of the W / O emulsion tend to become finer, and conversely, if the amount of surfactant used is low, the droplets of the W / O emulsion tend to become larger. Therefore, the average particle size of the aerogel can be adjusted by increasing or decreasing the amount of surfactant used.

[0033] When forming a W / O emulsion, known methods for forming W / O emulsions can be used to disperse the aqueous silica sol in a hydrophobic solvent. From the viewpoint of ease of industrial production, emulsion formation by mechanical emulsification is preferred, and specific examples include methods using a mixer, homogenizer, etc. A homogenizer can preferably be used.

[0034] Since the average particle size of silica sol droplets in a W / O emulsion generally corresponds to the average particle size of aerogel, the average particle size of aerogel can be controlled by controlling the droplet diameter.

[0035] Furthermore, by making the particle size of the silica sol droplets in the emulsion sufficiently small, the shape of the silica sol droplets becomes less likely to be disturbed, making it even easier to obtain spherical aerogels with higher circularity (however, the average particle size of the aerogel also becomes smaller).

[0036] <(3) Step of converting the W / O type emulsion into a dispersion of the gelled product> In this process, after forming an emulsion by the aforementioned operation, the aqueous silica sol is gelled. Any known gelling method can be used without particular limitation, as long as the emulsion state is not disrupted.

[0037] One preferred method is to adjust the pH during aqueous silica sol formation so that the time until gelation is sufficiently extended. In other words, the pH is adjusted so that gelation does not occur during emulsion formation of the silica sol, but then occurs after being held at a certain temperature for a certain period of time.

[0038] After adjusting to the gelation temperature, the time it takes for gelation to begin depends on the pH, gelation temperature, and silica sol concentration. Generally, the lower the pH, the lower the gelation temperature, and the lower the silica sol concentration, the longer the time tends to be. For example, at pH 5, a temperature of 50°C, and a silica concentration (SiO2 equivalent) of 80 g / L in the silica sol, it takes a few minutes. At pH 3, a temperature of 70°C, and a silica concentration (SiO2 equivalent) of 80 g / L in the silica sol, it takes about 60 minutes.

[0039] Another preferred method involves adding a basic substance to the emulsion to raise the pH of the W phase, making it weakly acidic or basic. In this case, it is preferable to prepare the metal oxide sol at a relatively stable low pH (around 0.5 to 2.5) when preparing it. A specific method for raising the pH of the W phase is to predetermine the amount of base needed to raise the W phase to the desired pH and then add that amount of base to the emulsion. The amount of base needed to achieve the desired pH can be determined by taking a fixed amount of the metal oxide sol used in the emulsion, measuring the pH of the taken metal oxide sol with a pH meter, adding the base used for gelation to the taken metal oxide sol, and measuring the amount of base needed to achieve the desired pH.

[0040] When adding a basic substance to an emulsion, it is preferable to prevent localized increases in pH by stirring with a mixer or similar device. Examples of basic substances include ammonia, caustic soda, and alkali metal silicates.

[0041] (4) A step of separating the dispersion into two layers: the O phase and the W phase. In the manufacturing method of the present invention, the dispersion of the gelled material prepared as described above is separated into an O phase and a W phase. After separation, the gelled material obtained in the above step is dispersed in the W phase.

[0042] As for the separation method, known methods for demuccaging emulsions can be used, but specifically, one or more methods selected from the following can be used in combination: addition of a water-soluble organic solvent, addition of a salt, application of centrifugal force, addition of an acid, and change in volume ratio (addition of water or a hydrophobic solvent). Preferably, a certain amount of water-soluble organic solvent can be added to the emulsion along with water as needed to separate it into the O phase and the W phase. After the separation step, generally the upper layer is the O phase (organic layer) and the lower layer is the W phase (aqueous layer). Examples of the above-mentioned water-soluble organic solvents include acetone, methanol, ethanol, and isopropyl alcohol. Of these, isopropyl alcohol can be preferably used because it is effective in improving the efficiency of the silylation treatment described later.

[0043] Furthermore, the addition of water is not necessarily required to form the W phase in this process. A method can be employed in which a sufficient amount of water from the water used as a raw material is discharged from the gelled material to allow for its dispersion. Specifically, this method can be implemented by selecting a water-soluble organic solvent that penetrates the pores of the gelled material and has the function of displacing water.

[0044] The amount of water-soluble organic solvent added is preferably adjusted according to the type and amount of surfactant used during emulsion formation. For example, when sorbitan monooleate is used as the surfactant for a W / O type emulsion, the O phase and W phase can be separated by adding a water-soluble organic solvent in an amount of about 0.1 to 0.4 times the mass of the O phase, stirring as necessary, and then allowing it to stand. However, in this case, it is preferable to add water along with the water-soluble organic solvent in an amount of about 0.6 to 0.9 times the mass of the O phase. Furthermore, the temperature during this separation operation is not particularly limited, but it can usually be carried out at around 20 to 70°C.

[0045] In carrying out the manufacturing method of the present invention, it is preferable to perform aging thereafter. This aging is carried out by adding a basic substance to the W phase (where the gelled material is dispersed) that has been separated from the O phase, and adjusting the pH of the W phase to weakly acidic or basic.

[0046] By adding a basic substance, the pH of the W phase, which is in an acidic range, rises, resulting in a weakly acidic or basic state. Specifically, the pH of the W phase is preferably 4.5 to 10, more preferably 5.5 to 8.5, and particularly preferably 6.0 to 8.0.

[0047] In the present invention, the basic substance used is a basic substance containing sodium. While inorganic bases such as sodium hydroxide, sodium bicarbonate, and sodium carbonate, and sodium salts of organic acids such as sodium acetate can be used as the basic substance, inorganic bases are preferred because organic acids may be undesirable impurities. Among these, sodium hydroxide is preferred because it allows for easy pH adjustment.

[0048] Furthermore, the gelled product can be matured by maintaining the maturation temperature at room temperature to approximately 80°C. The maturation time can be set appropriately depending on the pH of the W phase and the maturation temperature, but it is generally between 0.2 and 12 hours.

[0049] <(5) Step to remove the O phase> The O phase generated in the step of separating the O phase and W phase into two layers (4) above is removed. This is to improve the processing efficiency in the subsequent step of silylation treatment of the gelled body. The removal method is not particularly limited, but it can be easily achieved by removing the O phase from the two separated O phase and W phase, for example by decantation, and recovering the W phase.

[0050] Here, it is not necessary to completely separate and remove the O phase, but in order to efficiently perform the silylation treatment in the step of silylation treatment of the gelled body contained in the W phase, it is preferable that the proportion of the O phase that remains unremoved be as small as possible. Preferably, it should be 20% by mass or less relative to the amount of W phase (including the mass of the gelled body), and more preferably 10% by mass or less.

[0051] (6) Step of silylation treatment of the gelled material dispersed in the W phase with a mixture of concentrated sulfuric acid and siloxanes. In the method for producing silica aerogel powder of the present invention, a silylation treatment is performed using a mixture of concentrated sulfuric acid and siloxanes. The mixture of concentrated sulfuric acid and siloxanes is added to the gelled material dispersed in the W phase, and the solution is stirred at a predetermined temperature for a certain period of time to carry out silylation.

[0052] As for concentrated sulfuric acid, a concentration of 90-100% can be used, preferably 95% or higher. If the sulfuric acid concentration is less than 90%, the sulfuric acid and siloxanes will not be miscible, the cleavage reaction of siloxane bonds described later will not be promoted, and an improvement in the silylation treatment rate cannot be expected.

[0053] As siloxanes, compounds containing a siloxane bond within the molecule can be used. Known siloxanes include those represented by the following general formulas (2) and (3).

[0054] [ka]

[0055] [In formula (2), n represents a non-negative integer; R represents a hydrophobic group such as a hydrocarbon group; and R' represents hydrogen or a hydrocarbon group. Multiple Rs and R's may be the same or different.]

[0056] [ka]

[0057] [In formula (3), m represents an integer between 3 and 10; R represents a hydrophobic group such as a hydrocarbon group; and R' represents hydrogen or a hydrocarbon group. Multiple Rs and R's may be the same or different.]

[0058] In formula (2) above, n is preferably an integer from 0 to 8. R and R' are preferably hydrocarbon groups, more preferably hydrocarbon groups having 1 to 10 carbon atoms, even more preferably hydrocarbon groups having 1 to 4 carbon atoms, and particularly preferably methyl groups.

[0059] When treated with the siloxanes shown in formula (2), the number of hydroxyl groups bonded to the silica powder surface changes depending on the number of n in formula (2). For example, if n is 0: ≡MO-SiRR'2(4) [In equation (4), M represents the Si atoms forming the silica powder (the same applies in all subsequent equations).] This will result in the following combination.

[0060] Also, if n is 1 or greater: equation (4) and (≡MO-)2SiRR' (5) This results in the formation of a bond, and the hydroxyl group is silylated in this way.

[0061] Specific examples of siloxanes represented by formula (2) above include dimethylpolysiloxanes such as hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane, dodecamethylpentasiloxane, and tetradecamethylhexasiloxane, as well as methylphenyl silicone oil. Hexamethyldisiloxane and octamethyltrisiloxane are preferred due to their good reactivity, and hexamethyldisiloxane is more preferred.

[0062] In formula (3) above, m is an integer between 3 and 10, preferably between 3 and 5. R and R' are preferably hydrocarbon groups, and preferred groups include those similar to R and R' in formula (2). When silica powder is treated with the compound represented by formula (3) (hereinafter also referred to as cyclic siloxane), the bond represented by formula (5) above is formed on the silica surface in the gelled body.

[0063] Specific examples of cyclic siloxanes represented by formula (3) above include hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, tetradecamethylcycloheptasiloxane, hexadecamethylcyclooctasiloxane, octadecamethylcyclononasiloxane, and eicosamethylcyclodecasiloxane. In terms of good reactivity, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, and decamethylcyclopentasiloxane are preferred, and octamethylcyclotetrasiloxane is more preferred.

[0064] A mixture of concentrated sulfuric acid and siloxanes is obtained by mixing concentrated sulfuric acid and siloxanes.

[0065] The siloxanes listed in formula (2) or (3) are thought to be readily miscible with sulfuric acid, forming silyl sulfate esters represented by formula (6) or (7) in the mixture. RR'2Si-OSO3H (6) RR'Si-(OSO3H)2(7) [In formulas (6) and (7), R represents a hydrophobic group such as a hydrocarbon group; R' represents hydrogen or a hydrocarbon group; multiple R's and R's may be the same or different.] The sulfate groups in formulas (6) and (7) have high leaving ability and readily mix and react with water to generate silanol groups. It is thought that silylation proceeds through dehydration condensation of these silanol groups with silanol groups on the silica surface, but the miscibility of concentrated sulfuric acid with siloxanes promotes the generation of silanol groups, thus accelerating the silylation reaction.

[0066] The amount of siloxane used depends on the type, but for example, when performing silylation treatment with hexamethyldisiloxane, 0.03A to 0.15A parts by mass are used per 100 parts by mass of silica (where A is the specific surface area (m²) of silica). 2 The amount is ( / g). ) is preferred. More preferably, it is 0.04A to 0.13A parts by mass. If it is desired to adjust the amount of remaining silanol groups in silica after silylation, a smaller amount than the above range may be used.

[0067] When using hexamethyldisiloxane or octamethylcyclotetrasiloxane, it is desirable to use concentrated sulfuric acid in an amount 1.5 to 10 times the mass of the siloxane being used. This is because when the amount of sulfuric acid used is 1.5 times the mass of the siloxane or more, the formation of silyl sulfate esters proceeds sufficiently, and a homogeneous mixture of siloxane and concentrated sulfuric acid is formed, thus promoting the silylation reaction. Furthermore, if the amount of sulfuric acid is 10 times the mass of the siloxane or less, there is no risk of the hydrophilic organic solvent in the W phase reacting with the sulfuric acid. The endpoint of mixing depends on the type of siloxane and the stirring efficiency, but it is preferable to consider the point at which the exothermic reaction due to mixing subsides as the endpoint. For example, when using 46g of concentrated sulfuric acid and 8g of hexamethyldisiloxane or octamethyltetrasiloxane, the exothermic reaction subsides in about 5 minutes and the temperature of the mixture stops rising.

[0068] There are various methods for adding a mixture of concentrated sulfuric acid and siloxanes to the W phase in which the gelled material is dispersed. For example, it can be added to the W phase in the reaction vessel at an appropriate rate using a funnel while stirring. If the internal temperature may exceed the temperature range described later due to the heat generated by mixing the concentrated sulfuric acid and the W phase, it is preferable to adjust the addition rate by using a dropping funnel or the like.

[0069] Furthermore, it is preferable to set the pH of the reaction solution to 0 to 1.0 in order to increase the efficiency of the silylation reaction and shorten the reaction time. The pH can be adjusted within this range using the sulfuric acid contained in the mixture of concentrated sulfuric acid and siloxanes, but if it falls below this range, an additional acidic component may be added. The additional acidic component is not limited to sulfuric acid, and any mineral acid or organic acid can be used. These mineral acids and organic acids include sulfuric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, thiocyanic acid, phosphoric acid, formic acid, acetic acid, trifluoroacetic acid, etc., with sulfuric acid and hydrochloric acid being more preferred.

[0070] The temperature during the silylation treatment is preferably 50 to 70°C, and more preferably 60 to 70°C. If the temperature exceeds this range, depending on the type of siloxane and hydrophilic organic solvent, these may distill off, potentially resulting in insufficient silylation. If the treatment is carried out at a temperature exceeding the above range, it is preferable to take measures to suppress the distillation off of siloxanes and hydrophilic organic solvents by using a condenser.

[0071] The time required for silylation treatment varies depending on the specific surface area of ​​the silica, the type of siloxane, the temperature, and the hydrophobicity of the target silica powder. Hydrophobicity can be expressed as the M value, which is the methanol concentration in which the silica powder is suspended, and this can also be considered an indicator of the progress of the silylation reaction. For example, when using 73 parts by mass of hexamethyldisiloxane for 100 parts by mass of silica with a specific surface area of ​​900-1000 m2 / g, and at a treatment temperature of 60°C, the time required to obtain hydrophobic silica with an M value of around 45 Vol% is 1 hour.

[0072] <(7) Step of extracting the gelled product silylated with a hydrophobic organic solvent> Since the W phase after silylation treatment contains non-volatile sulfuric acid, it is preferable to remove the acidic components, and known methods such as neutralization treatment and water rinsing can be employed. Among these, the method of neutralization treatment by adding a basic substance is preferred. By adding a basic substance, the pH of the W phase is brought to a state where it is neutral to weakly acidic, and specifically, the pH of the W phase is preferably 1.0 to 7.5, and more preferably 1.5 to 7.0.

[0073] Any water-soluble basic substance can be used, including inorganic bases such as hydroxides, carbonates, bicarbonates, and aqueous ammonia, and organic salts such as acetates.

[0074] Furthermore, this neutralization treatment can be carried out by maintaining a temperature of 35°C to 80°C. Since an exothermic acid-base neutralization reaction occurs in this process, this temperature range can be maintained without additional heating. The time required to add the basic substance can be appropriately set depending on the temperature of the W phase, but it is typically 0.5 to 1 hour.

[0075] After the neutralization treatment described above, the gelled substance is extracted into a hydrophobic organic solvent. By removing the aqueous layer formed by the extraction of the gelled substance into the hydrophobic organic solvent, most of the salts can be removed. The gelled substance extracted into the hydrophobic organic solvent is dispersed in the hydrophobic organic solvent. Any hydrophobic organic solvent can be used for extracting the gelled substance, but it is preferable to use one with a relatively low boiling point that is easily removed by distillation during drying and that is non-reactive with concentrated sulfuric acid used when recovering siloxanes, as described later. Hexane, heptane, nonane, decane, etc., can be used, and hexane, heptane, and decane can be used more preferably.

[0076] After extraction into the hydrophobic organic solvent as described above, it is preferable to remove any remaining salts, sulfuric acid residues, etc., from the gelled material. If sulfuric acid remains in the gelled material, in the step of recovering silica gel (8) described later to obtain hydrophobic spherical silica aerogel, the concentrated sulfuric acid will decompose the silyl groups on the surface of the gelled material, causing drying shrinkage of the gelled material, making it difficult to obtain aerogel. To remove the salts and mineral acids, it is preferable to wash the hydrophobic organic solvent with an aqueous solution of water or alcohol, and this washing operation can be carried out by known methods. To improve washing efficiency, it is preferable to use an aqueous solution of isopropyl alcohol at a concentration of about 10 to 60 wt%. Furthermore, it is preferable to raise the temperature within a range that does not exceed the boiling point of the hydrophobic organic solvent in order to improve washing efficiency. This can usually be done in the range of 45 to 70°C. This washing operation can be carried out by adding an aqueous solution of water or alcohol at a volume of about one-quarter to two times the volume of the hydrophobic organic solvent used to extract the gelled material to the hydrophobic organic solvent from which the gelled material obtained in this step was extracted, and stirring and mixing while the organic phase and aqueous phase are separated. In this case, it is preferable to repeat the washing operation until the pH of the aqueous phase after washing is 4 or higher and the electrical conductivity is 200 μS / cm or lower.

[0077] (8) Step of recovering silica gel and obtaining hydrophobic spherical silica aerogel. In the step of extracting the gelled product silylated with a hydrophobic organic solvent as described in (7) above, the gelled product dispersed in the hydrophobic organic solvent obtained can be filtered off and the hydrophobic organic solvent removed (i.e., dried) to obtain a hydrophobic spherical silica aerogel. The drying temperature is preferably above the boiling point of the solvent and below the decomposition temperature of the surface treatment agent, and the pressure is preferably at normal pressure or reduced pressure.

[0078] When silica aerogel powder made of the hydrophobic spherical silica aerogel of the present invention is produced by the method described above, it exhibits hydrophobicity. However, it is also possible to change it to hydrophilicity by thermally decomposing the hydrophobic groups on the surface. For example, the hydrophobic groups on the surface can be thermally decomposed by holding it at a temperature of 400 to 700°C, preferably 500 to 600°C, for about 1 to 8 hours in a non-oxidizing atmosphere (such as a nitrogen atmosphere).

[0079] <(9)(8) Step of recovering siloxanes from a hydrophobic organic solvent containing siloxanes generated in the step of recovering silica gel and obtaining hydrophobic spherical silica aerogel> In the above step (8) recovering silica gel and obtaining hydrophobic spherical silica aerogel, the filtrate obtained when filtering the gelled product and the distillate obtained when drying the gelled product are hydrophobic organic solvent solutions containing siloxanes. Since siloxanes are relatively expensive reagents and their use significantly impacts manufacturing costs, it is preferable to recover the siloxanes from this hydrophobic organic solvent solution containing siloxanes.

[0080] Methods for recovering siloxanes from hydrophobic organic solvent solutions containing siloxanes include distillation isolation using boiling point differences, but siloxanes can also be recovered by contacting the hydrophobic organic solvent solution containing siloxanes with concentrated sulfuric acid.

[0081] When a hydrophobic organic solvent solution containing siloxanes is brought into contact with concentrated sulfuric acid, the siloxanes can be extracted into the concentrated sulfuric acid, and the siloxanes can be recovered as a mixture of concentrated sulfuric acid and siloxanes.

[0082] The amount of concentrated sulfuric acid used relative to siloxanes depends on the type of siloxane, but when recovering hexamethyldisiloxane or octamethylcyclotetrasiloxane, it is preferable to use concentrated sulfuric acid at least one times the mass of the siloxane contained in the hydrophobic organic solvent solution, from the viewpoint of improving the recovery rate of siloxanes. Although the recovery rate of siloxanes does not change much even if 1.5 times or more concentrated sulfuric acid is used, it is more preferable to use 1.5 times or more concentrated sulfuric acid to ensure handling and guarantee the frequency of liquid-liquid contact. There is no upper limit on the amount used, but from the viewpoint of reducing environmental impact, it is preferable to avoid using more concentrated sulfuric acid than necessary, by determining the amount of concentrated sulfuric acid that ensures timely handling according to the experimental and manufacturing scale.

[0083] As will be described later, the siloxanes recovered as a mixture of concentrated sulfuric acid and siloxanes can be used for the wet silylation of metal oxides under acidic conditions. In this case, the acid used in silylation can be concentrated sulfuric acid, and the upper limit of the amount of concentrated sulfuric acid used in the extraction of siloxanes can be determined by taking into account the amount of concentrated sulfuric acid used in silylation.

[0084] The mixing temperature described above can be within the range of 10 to 90°C. Above 90°C, there is a concern that volatilization may occur depending on the type of siloxane. At even higher temperatures, concentrated sulfuric acid may act as an oxidizing agent, potentially oxidizing the siloxanes and hydrophobic organic solvents. Below 10°C, the degree of phase separation between the organic layer and the sulfuric acid layer may deteriorate.

[0085] The degree of siloxane extraction can be determined by measuring the siloxanes in the hydrophobic organic solvent solution using gas chromatography or liquid chromatography. The extraction time varies depending on the conditions, but for example, with 80g of heptane containing 3% hexamethyldisiloxane and 45g of concentrated sulfuric acid, stirring for about 5 minutes will reduce the siloxanes in the hydrophobic organic solvent solution to the order of a few ppm.

[0086] Siloxanes recovered as a mixture with concentrated sulfuric acid by extracting siloxanes from a hydrophobic organic solvent solution containing siloxanes with concentrated sulfuric acid can be used for the wet silylation of metal oxides under acidic conditions. For example, in a method for producing hydrophobic silica powder, siloxanes can be used as siloxanes for silylation of silica powder to obtain hydrophobic silica powder. That is, instead of adding acid and siloxanes for silylation treatment, silylation treatment can be performed by adding a mixture of concentrated sulfuric acid and siloxanes, thereby obtaining hydrophobic silica powder.

[0087] For example, this mixture of concentrated sulfuric acid and siloxanes can be used again in the process of silylation treatment of the gelled body dispersed in the (6)W phase with the mixture of concentrated sulfuric acid and siloxanes, and can be used in at least a portion of the mixture. The recovered mixture of concentrated sulfuric acid and siloxanes may be used as the entire amount of the mixture of concentrated sulfuric acid and siloxanes in the process of silylation treatment of the gelled body dispersed in the (6)W phase with the mixture of concentrated sulfuric acid and siloxanes, or as a portion of it.

[0088] Furthermore, siloxanes can also be isolated by adding a mixture of concentrated sulfuric acid and siloxanes obtained by extraction to water, thereby hydrating the concentrated sulfuric acid. When isolating siloxanes extracted with concentrated sulfuric acid, the mixture of concentrated sulfuric acid and siloxanes is added to water, and the sulfuric acid molecules are hydrated, releasing the siloxanes and separating them into two layers. The upper layer contains the released siloxanes, which can then be separated by decantation.

[0089] The amount of water added for hydration should preferably be 1.5 parts by mass or more relative to the concentrated sulfuric acid. If the amount is 1.5 parts by mass or more, the sulfuric acid molecules will be completely hydrated, and the sulfuric acid layer will lose its ability to dissolve siloxanes, allowing for the recovery of siloxanes in good yield.

[0090] The siloxanes obtained by liberation do not retain their structure before concentrated sulfuric acid extraction, except when the siloxanes extracted into the sulfuric acid layer are hexamethyldisiloxanes. For example, when a hydrophobic organic solvent containing a dialkylsiloxane is brought into contact with concentrated sulfuric acid, the structure of the siloxane extracted into the concentrated sulfuric acid layer is considered to be dissolved in the form of silyl sulfate esters represented by formulas (6) and (7).

[0091] When a sulfuric acid layer containing the silyl sulfate ester shown in formula (7) is hydrated, the liberated siloxanes are molecules produced by the hydrolysis and condensation polymerization of the silyl sulfate ester, and are a mixture of various linear and cyclic siloxanes. On the other hand, when hexamethyldisiloxane, which does not have a dimethylsilyl group and only has a trimethylsilyl group, is extracted into concentrated sulfuric acid, it dissolves in the concentrated sulfuric acid layer only in the form of formula (6), and the siloxane obtained by hydration is hexamethyldisiloxane, in which the trimethylsilyl group is ether-bonded.

[0092] The isolated siloxanes can be used in applications where siloxanes are typically used. For example, in a method for producing hydrophobic silica powder, they can be used as siloxanes to silylate silica powder to obtain hydrophobic silica powder. In the step of silylation treatment of the gelled body dispersed in the (6)W phase with a mixture of concentrated sulfuric acid and siloxanes, they can be used as at least a portion of the siloxanes to form a mixture with concentrated sulfuric acid. [Examples]

[0093] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The examples and comparative examples were evaluated using the following method.

[0094] <Evaluation Method> The silica aerogel powder, consisting of hydrophobic spherical silica aerogels produced in the examples and comparative examples, was tested for the following items.

[0095] (D50) Silica aerogel powder was added to ethanol and ultrasonically dispersed for 30 minutes. The resulting ethanol dispersion was measured using a Beckman Coulter Multisizer 3 precision particle size distribution analyzer, with the volume-based cumulative 50% diameter (D50) measured using a 100 μm aperture tube.

[0096] (Specific surface area, pore volume, and oil absorption capacity) The BET specific surface area was obtained by acquiring adsorption isotherms and analyzing them using the BET method, similar to the measurement of BJH pore volume described above. The partial pressure (P / P0) range during the analysis was 0.1 to 0.25. The adsorption isotherms used in the BET and BJH methods were acquired using BELSORP-max manufactured by Nippon Bell Co., Ltd. Oil absorption was measured according to JIS K6217-4 "Method for determining oil absorption".

[0097] (M value) Hydrophobic silica powder floats in water but completely suspends in methanol. Using this property, the M value, measured by the following method, was used as an indicator of silylation treatment by the hydrophobic groups on the silica surface. Silica powder that suspends in water without the addition of methanol using the following method is hydrophilic silica powder, and its M value is 0. Silica powder with an M value of 1 or higher is hydrophobic silica powder and floats in water without methanol. A larger M value indicates that more methanol is required for the silica powder to suspend in water, thus indicating higher hydrophobicity.

[0098] 0.2 g of hydrophobic silica powder was added to 50 ml of water in a 200 ml beaker and stirred with a magnetic stirrer. Methanol was then added using a burette, and the mixture was added dropwise until the entire amount of hydrophobic silica powder was wet and suspended in the solvent in the beaker. During this process, methanol was guided into the solution via a tube to prevent direct contact with the sample. The volume percentage of methanol in the methanol-water mixture at the endpoint was defined as the hydrophobicity (M value). M value = methanol drop volume / (methanol drop volume + 50 ml) × 100

[0099] (Average circularity) Silica aerogel powder was observed using a Hitachi High-Technologies SEM (S-5500) with an acceleration voltage of 3.0 kV, secondary electron detection, and a magnification of 1000x. The circularity of the silica aerogel particles was calculated by image analysis of the obtained SEM images using the following formula. The average circularity was calculated by averaging the circularity C value for more than 2000 silica aerogel particles. C = 4πS / L 2 [In the above formula, S represents the area (projected area) occupied by the particle in the image. L represents the length of the outer edge of the particle in the image (perimeter).]

[0100] (Carbon content) The carbon content was measured using an elemental analyzer (vario MICRO cube) manufactured by Elementor Japan Co., Ltd.

[0101] <Method for measuring siloxanes> The concentrations of siloxanes in hydrophobic organic solvents were measured by gas chromatography. The measuring equipment and conditions used are shown below.

[0102] Equipment: Shimadzu GC-2014 Detector: FID (Flaming Ion Detector) Capillary column: Agilent J&W DB-1, Length: 60m, Inner diameter: 0.32mm, Film thickness: 5μm Column temperature: 200℃ Inlet pressure: 100kPa Column flow rate: 0.89 mL / min Split ratio: 8.4 Injection mode: Split (manual injection) Control mode: Pressure control Evaporation chamber: 250℃ Detector: 250℃

[0103] <Example 1> While stirring 100g of sulfuric acid with a stirring blade, 100g of sodium silicate was gradually added to prepare an aqueous silica sol. At this time, the pH was 2.9.

[0104] To 139 g of the aqueous silica sol prepared above, 129 g of heptane was added, and 1.5 g of sorbitan monooleate was added. This solution was stirred using a homogenizer at 4600 rpm for 2.5 minutes to form a W / O emulsion.

[0105] The obtained W / O emulsion was gelled at 70°C for 60 minutes while being stirred with a stirring blade. Subsequently, 71 g of isopropyl alcohol and 58 g of deionized water were added, and the O phase and W phase were separated while being stirred with a stirring blade. Next, 9.63 g of 0.5 mol / L sodium hydroxide aqueous solution was added to the W phase. At this time, the pH of the W phase was 7.1. The gelled body was aged at 70°C for 10 minutes. The W phase was recovered by removing the O phase by decantation. The silica concentration of the W phase was 4.2 wt%.

[0106] 45.5 g of concentrated sulfuric acid was mixed with 7.7 g of hexamethyldisiloxane (HMDSO) and stirred for 5 minutes to obtain a mixture. This mixture was added to the W phase described above, and a silylation treatment was carried out at 60°C for 15 minutes while stirring. The pH of the solution during the silylation treatment was 0.

[0107] After silylation, 130 g of 24% sodium hydroxide aqueous solution was added while stirring with a stirring blade to perform neutralization. The pH at this time was 2.0.

[0108] Next, 90g of heptane was added, the gelled material was extracted, and the aqueous layer was removed by decantation to recover the hydrophobic organic solvent layer. The hydrophobic organic solvent layer was washed twice with 129g of 55% isopropyl alcohol.

[0109] After washing, the gelled material was filtered off using a suction filter. The gelled material was dried by heating at -100 kPa and 150°C for more than 16 hours to obtain silica aerogel powder. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0110] <Comparative Example 1> The procedure was the same as in Example 1 up to the step of removing the O phase after maturation of the gelled product. In the silylation treatment step, 45.5 g of concentrated sulfuric acid and 7.7 g of HMDSO were added separately to the W phase, and the silylation treatment was carried out at 60°C for 15 minutes while stirring. The pH of the solution during the silylation treatment was 0.

[0111] After silylation, 130 g of 24% sodium hydroxide aqueous solution was added while stirring with a stirring blade to perform neutralization. The pH at this time was 2.0. Next, 90g of heptane was added to attempt to extract the gel, but it did not disperse in the heptane. The W phase was transferred to a funnel, the gel was filtered off, and 1500g of pure water was added to wash away the salts. Subsequently, 300g of isopropyl alcohol was added, and the resulting gel was dried by heating at -100kPa and 150°C for more than 16 hours to obtain silica aerogel powder. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0112] <Example 2> The same procedure as in Example 1 was followed, except that the silylation treatment time was set to 30 minutes. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0113] <Comparative Example 2> The procedure was the same as in Example 1 up to the step of removing the O phase after maturation of the gelled product. In the silylation treatment step, 45.5 g of concentrated sulfuric acid and 7.7 g of HMDSO were added separately to the W phase, and the silylation treatment was carried out at 60°C for 30 minutes while stirring. The pH of the solution during the silylation treatment was 0.

[0114] After silylation, 130 g of 24% sodium hydroxide aqueous solution was added while stirring with a stirring blade to perform neutralization. The pH at this time was 2.0. Next, 90g of heptane was added, the gelled substance was extracted, and the W phase was removed by decantation to recover the O phase. The O phase was washed twice with 129g of 55% isopropyl alcohol.

[0115] The silica slurry after washing was filtered using a suction filter. The gelled material was dried by heating at -100 kPa and 150°C for more than 16 hours to obtain silica aerogel powder. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0116] <Example 3> The same procedure as in Example 1 was followed, except that the silylation treatment time was set to 60 minutes. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0117] <Comparative Example 3> The procedure was the same as in Comparative Example 2, except that the silylation treatment was performed at 60°C for 60 minutes. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0118] <Example 4> The same procedure as in Example 1 was followed, except that the silylation treatment time was set to 180 minutes. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0119] <Example 5> While stirring 100g of sulfuric acid with a stirring blade, 100g of sodium silicate was gradually added to prepare an aqueous silica sol. At this time, the pH was 2.9.

[0120] To 139 g of the aqueous silica sol prepared above, 129 g of heptane was added, and 1.5 g of sorbitan monooleate was added. This solution was stirred using a homogenizer at 4600 rpm for 2.5 minutes to form a W / O emulsion.

[0121] The obtained W / O emulsion was gelled at 70°C for 60 minutes while being stirred with a stirring blade. Subsequently, 71 g of isopropyl alcohol and 58 g of deionized water were added, and the O phase and W phase were separated while being stirred with a stirring blade. Next, 9.63 g of 0.5 mol / L sodium hydroxide aqueous solution was added. At this time, the pH of the W phase was 7.1. The gelled body was aged at 70°C for 10 minutes. The W phase was recovered by removing the O phase by decantation. 33.9 g of concentrated sulfuric acid was added to the W phase. The silica concentration of the W phase after the addition of concentrated sulfuric acid was 3.7 wt%.

[0122] A mixture was obtained by adding 7.7 g of HMDSO to 11.6 g of concentrated sulfuric acid and stirring for 5 minutes. This mixture was added to the W phase described above, and a silylation treatment was carried out at 60°C for 60 minutes while stirring. The pH of the solution during the silylation treatment was 0.

[0123] After silylation, 130 g of 24% sodium hydroxide aqueous solution was added while stirring with a stirring blade to perform neutralization. The pH at this time was 2.0.

[0124] Next, 90g of heptane was added, the gelled substance was extracted, and the W phase was removed by decantation to recover the O phase. The O phase was washed twice with 129g of 55% isopropyl alcohol.

[0125] The silica slurry after washing was filtered using a suction filter. The gelled material was dried by heating at -100 kPa and 150°C for more than 16 hours to obtain silica aerogel powder. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0126] <Example 6> The procedure was carried out in the same manner as in Example 1 up to the point of washing the heptane slurry containing the silylated gel with 55% isopropyl alcohol, and the solvent was removed from the resulting silica slurry using an evaporator. The resulting residue was 125 g and contained 3.9% HMDSO.

[0127] 110 g of the distillate (containing 4.3 g of HMDSO) was added to 45.5 g of concentrated sulfuric acid and stirred at 25°C for 5 minutes. After standing for 1 minute, the organic layer was removed by decantation, and a sulfuric acid layer, a mixture of concentrated sulfuric acid and HMDSO, was obtained, from which HMDSO had been extracted by concentrated sulfuric acid. At this time, the amount of HMDSO remaining in the organic layer was 20 ppm or less. After adding 3.4 g of fresh HMDSO to this sulfuric acid layer, it was added as a mixture of concentrated sulfuric acid and HMDSO to be used during the silylation treatment in the manufacturing process of the silica aerogel of Example 1, and the other operations were carried out in the same manner as in Example 1. The silylation conditions and the physical properties of the obtained silica aerogel powder are shown in Table 1.

[0128] <Example 7> The procedure was carried out in the same manner as in Example 1 up to the point of washing the heptane slurry containing the silylated gel with 55% isopropyl alcohol, and the solvent was removed from the resulting silica slurry using an evaporator. The resulting residue was 125 g and contained 3.9% HMDSO.

[0129] 120 g of the distillate (containing 4.7 g of HMDSO) was added to 45.5 g of concentrated sulfuric acid and stirred at 25°C for 5 minutes. After standing for 1 minute, the organic layer was removed by decantation, and a sulfuric acid layer, a mixture of concentrated sulfuric acid and HMDSO, was obtained, from which HMDSO had been extracted by concentrated sulfuric acid. At this time, the amount of HMDSO remaining in the organic layer was 20 ppm or less. The obtained sulfuric acid layer was slowly added to 90 g of water to hydrate the sulfuric acid molecules. The HMDSO liberated by hydration was separated by decantation. The amount obtained was 4.2 g, and the yield for the isolation of HMDSO from the distillate was 90%.

[0130] [Table 1]

[0131] Comparing the results of Example 1 and Comparative Example 1, it can be seen that there is a significant difference in the M value (indicating hydrophobicity) and carbon content depending on whether or not concentrated sulfuric acid and HMDSO are mixed before addition. In Comparative Example 1, when not mixed before addition, silylation is insufficient, and the pores of the gelled material cannot be completely replaced with a hydrophobic organic solvent, resulting in shrinkage during drying and low oil absorption. Furthermore, even with a processing time of 1 hour, the M value is 30 Vol% or less when there is no prior mixing. These results indicate that mixing concentrated sulfuric acid and HMDSO before addition accelerates the silylation process. In Example 6, it can be seen that unreacted HMDSO can be recovered as a mixture of concentrated sulfuric acid and HMDSO by extracting it with concentrated sulfuric acid, and that this mixture can be reused in the next production. In Example 7, it can be seen that after extracting unreacted HMDSO with concentrated sulfuric acid to obtain a sulfuric acid layer, which is a mixture of concentrated sulfuric acid and HMDSO, HMDSO can be isolated and recovered by hydrating the sulfuric acid in this sulfuric acid layer.

Claims

1. (1) Steps to prepare aqueous silica sol (2) A step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W / O type emulsion. (3) A step of converting the W / O type emulsion into a dispersion of the gelled product. (4) A step of separating the dispersion into two layers, the O phase and the W phase. (5) Step to remove the O phase (6) A step of silylation treatment of the gelled body dispersed in the W phase with a mixture of concentrated sulfuric acid and siloxanes. (7) Step of extracting the gelled product that has been silylated with a hydrophobic organic solvent. (8) Steps to recover silica gel and obtain hydrophobic spherical silica aerogel A method for producing silica aerogel powder comprising the above.

2. Furthermore, (9) (8) A step of recovering siloxanes from a hydrophobic organic solvent solution containing siloxanes that is generated in the step of recovering silica gel and obtaining hydrophobic spherical silica aerogel. A method for producing silica aerogel according to claim 1, comprising:

3. The step of recovering siloxanes from a hydrophobic organic solvent solution containing siloxanes that is generated in the step of recovering silica gel in (9)(8) above to obtain hydrophobic spherical silica aerogel is a step of recovering siloxanes by extracting siloxanes from a hydrophobic organic solvent solution containing siloxanes with concentrated sulfuric acid. The method for producing silica aerogel according to claim 2.

4. A method for producing silica aerogel according to claim 3, wherein the siloxanes are obtained as a mixture of concentrated sulfuric acid and siloxanes by recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel by by recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel by recovering the silica gel in the step of recovering the silica gel in the step of recovering the silica gel by recovering the silica gel in the step of recover

5. A method for producing spherical silica aerogel powder according to claim 3, wherein in the step of recovering silica gel according to (9)(8) above to obtain hydrophobic spherical silica aerogel, the siloxanes are recovered from a hydrophobic organic solvent solution containing siloxanes that is generated in the step of recovering silica gel to obtain hydrophobic spherical silica aerogel, by extracting the siloxanes in concentrated sulfuric acid and then adding the concentrated sulfuric acid to water.

6. A method for producing silica aerogel according to claim 5, wherein after obtaining siloxanes by recovering siloxanes from a hydrophobic organic solvent solution containing siloxanes generated in the step of recovering silica gel (9) (8) to obtain hydrophobic spherical silica aerogel, the siloxanes are used as at least a portion of the siloxanes in the step of silylation treatment of a gelled body dispersed in the W phase (6) with a mixture of concentrated sulfuric acid and siloxanes.

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